1. Description of the Related Art
A common, well established method of transferring semiconductor devices from their original substrate to a new substrate is known as the Epitaxial Liftoff Processs (ELO) in which the devices are fabricated on top of a sacrificial release layer. The ELO method is disclosed in U.S. Pat. No. 5,641,381 to Bailey, et. al. and U.S. Pat. No. 4,846,931 to Gmitter, et. al.
According to the ELO method, by attaching the top of the semiconductor devices to an adhesive or a wax layer, the release layer is chemically etched thus releasing the devices from their original substrate. The released devices are then attached to the new substrate using another adhesive.
Both patents mentioned above teach a technique of applying a layer of a polymer or a wax on top of a device and curing this layer at specific temperature. As a result compressive stress develops in the device layer and tensile stress--in the polymer or wax layer leading to the curling up of the film as the sacrificial layer is etched away thus releasing both the device layer and the polymer/wax layer together from the substrate. The Gmitter patent is the original ELO patent and teaches this technique in general while the Bailey patent develops it further and describes specific means of removing InP layers by using a ternary InGaAs sacrificial layers.
The ELO method has serious disadvantages. It leads to a loss of the original interface between the bottom of the removed devices and the release layer which can negatively effect their ultimate electrical characteristics. This disadvantage is particularly acute if the device layer is very thin.
Other disadvantages are the difficulty in positioning the removed devices on specific locations on the new substrate to maintain registration between the devices and the inability in transferring prefabricated metallization between the devices, such as in active matrix arrays. In addition, the device layers released according to the ELO method are only supported by a thin and flexible polymer/wax layer often resulting in cracks and fractures in the thin device layer before bonding thus greatly reducing the fabrication yield.
Finally, if the devices and/or circuits on the device layer are to be electrically isolated by forming islands in the device layer prior to transfer, the registration between these devices and/or circuits will be lost if the transfer is according to the ELO technique.
The inventor is unaware of any method which overcomes the above mentioned problems associated with the ELO method. Yet there is a need for a transfer method which will:
(a) maintain the original electrical interfaces between the device layer and the layer on top of which the device layer is fabricated; PA1 (b) maintain the original registration between the individual devices; PA1 (c) need no special bonding layer between the bottom of each device and the new substrate; and PA1 (d) maintain electrical interconnections between individual devices prefabricated prior to transfer.
The method proposed in this inventions accomplishes all these tasks. As a result, the devices and circuits will operate under conditions identical to those prevalent at the pre-transfer stage without any need for further processing (other than deposition of light modulating or emitting layers in case of displays).